Modern technology for producing microelectronic and optoelectronic circuits allows the efficient production of microlasers. Diode semi-conducting microlasers are especially well known, as they are the most frequently used type of lasers. When charged with electricity the thin layer around the semi-conducting p-n junction functions as an active medium emitting light. This active laser region has to be within the resonant cavity that can be made in several different ways. With respect to the type of resonator, the main types of semi-conducting lasers are Fabry-Pérot, DFB and VCSEL.
The Fabry-Perot laser is the simplest and currently the most widely used. Its resonant cavity is made by breaking the semi-conductor crystals containing the p-n junction on both sides and along the crystal structure. In this way we create two completely flat, parallel surfaces functioning as mirrors. The light travels along the p-n junction, being reflected several times from these surfaces before leaving the diode as a laser beam.
In the case of DFB (distributed feedback) lasers the light also travels along the junction. Here, the mirrors on both ends of the diode are made by etching a diffraction grating into it. This diffraction grating has a periodic structure of high and low refractive indexes known as a one-dimensional Bragg mirror, which can, due to the interference of light, selectively reflect a specific wavelength—the one at which the laser emits light. In this way we create better mirrors than in the case of Fabry-Pérot lasers, as here the laser has a narrower spectral line.
Unlike the above-mentioned lasers the VCSEL (vertical-cavity surface-emitting laser) emits the laser light perpendicular to the p-n junction. In this case the resonant cavity has to be made in such a way that its resonant effect is the largest in the direction perpendicular to the junction; for this reason, the mirrors are in the junction plane. The mirrors are made by the alternate deposition of flat layers of solids with alternating, high and low refractive indexes. The physical principle of reflection is the same as in the case of DFB lasers, only that here the direction and the production method are different.
All the above types of the resonant cavity are one-dimensional optical microresonators that can also be named linear microresonators. These limit the light waves to only one direction, i.e., to the direction perpendicular to the mirrors.
Furthermore, production methods for microlasers based on whispering-gallery microresonators are known. In this case small transparent spheres or droplets can behave as optical resonators. If the refractive index of the medium inside the droplet is larger than the external refractive index, the light can totally reflect off the boundary back to the droplet. In this way we get circular orbits of light reflecting many times and totally off the surface and coming back to the same point. If the length of an orbit equals multiple wavelengths, the resonant condition is met and the droplet functions as an optical microresonator. Usually, the light source in a resonator is simply a fluorescent dye dispersed in the droplet and pumped with the external light. The light spectrum emitted by a droplet includes the resonance peaks corresponding to the circular resonance orbits. If a droplet is pumped with a pulsed laser and the dye is such that it has an effect of stimulated emission, the threshold for laser operation is exceeded. Now the resonator emits single or multi-mode light.
From the reference literature we have been, for some time, also well acquainted with the methods of producing dye lasers based on cholesteric (chiral nematic) liquid crystals and on liquid-crystal blue phases. A review of dye lasers based on cholesteric liquid crystals doped with a laser dye is given in the review article by Harry Coles and Stephen Morris, Liquid-crystal lasers, Nature Photonics, Vol. 4, 676-685, (2010). The main principle of the functioning of a dye laser based on cholesteric liquid crystals is based on a one-dimensional helical structure of the cholesteric phase that is formed spontaneously and is characteristic of this phase. Due to a large optical anisotropy, i.e., the difference between the rates of light propagation along and across the cholesteric molecules, the helical structure of the molecules in the cholesteric phase represents an optical medium, whose refractive index is periodically changed along the helix. This medium thus spontaneously creates a one-dimensional, optically modulated structure, the period of which is between the orders of 100, nm and 100 ,μm and can be altered by selecting a substance or by mixing several different substances. The consequence of the one-dimensional modulation of the refractive index is the appearance of the forbidden band in the dispersion relation for the propagation of light along the helix, also called the photonic bandgap. The propagation of the light, the frequency of which is in the forbidden frequency band, is not allowed in such a substance. It is a special characteristic of such a substance that the light falling on cholesteric liquid crystals, in the direction of the helix, reflects if its frequency (and indirectly its wavelength) falls in the forbidden dispersion band. The cholesteric phase, thus, creates one-dimensional (1D) photonic crystals. Such 1D photonic crystals can be used as Bragg mirrors that limit the space and create a 1D optical laser resonator. We also know of special variations of Bragg mirrors based on the cholesteric phase, where we use a pair of identical cholesteric mirrors, while putting a thin dielectric layer between them. Such a structure also creates a 1D optical laser resonator, where the laser's functioning is achieved by doping the liquid crystals or the thin dielectric layer with the laser dye. All such dye lasers based on cholesteric liquid crystals emit coherent laser light in a precisely determined direction.
In the reference literature we can find two technical solutions for a 3D spherical laser emitting coherent laser light evenly to all directions in space. In the patent-registration documentation U.S. Pat. No. 4,829,537,, Th. M. Baer describes a technical solution for a spherical laser based on a spherical resonator made from a solid, laser-active substance. The spherical laser resonator is shaped as an isotropic sphere, produced from a laser-active material, and coated with a thin reflective layer. The optical transmittance of this reflective layer is made in such a way that it transmits all of the light, with which we can pump, through an external source, the laser-active material from the spherical resonator, while at the same time, this layer strongly reflects the wavelengths of its own electromagnetic oscillation modes formed inside the spherical resonator. The above patent registration also describes various ways of optical coupling to the external pumping light source and the pumping of the active medium, like using optical fibres or a prism. The author of the invention gives an example of a technical solution, i.e., the Nd:YAG tiny spheres optically pumped with a diode laser. The above registration documentation does not offer any solution for an obvious problem of aligning the frequencies of its EM oscillation modes of the laser-active sphere determined with the sphere's radius, and its frequency of the stimulated emission determined by the characteristics of the active medium, in this case the Nd:YAG material. The technical weakness of the proposed spherical laser is thus the large temperature sensitivity of the amplitude of the stimulated emission of the spherical resonator with a surface reflector, which is a result of the resonator's temperature elongation.
The patent-registration documentation US 2006/0227842, A1, prepared by S. S. Townsend and R. LaComb describes a technical realisation of a spherical laser similar to the one described in the patent documentation U.S. Pat. No. 4,829,537. The authors describe a spherical transparent bowl filled with an active laser medium. The inside surface of the spherical bowl is coated with a partly reflective layer, so that the bowl can form a spherical optical resonator. The above authors explain that the stimulated emission of the active laser medium filling the resonator is induced by external influences. When the stimulated emission overcomes the losses in the resonator, we obtain an evenly distributed and emitted laser light. The authors also describe a technical solution, whereby a reflective sphere is placed in the middle of the bowl—the resonator—while the active medium fills the space forming a shell between the external bowl and the sphere in the centre of the bowl.
From the reference literature we are also well acquainted with the research into the mixtures of liquid crystals and an isotropic liquid forming special types of substances called Polymer Dispersed Liquid Crystals, abbreviated as PDLCs. In these mixtures liquid crystals and isotropic liquids do not react, so that liquid crystals spontaneously separate from the mixture forming tiny droplets. For the case of nematic liquid crystals we know of different molecule structures of liquid crystals in a droplet; we also know the structures of the droplets formed by cholesteric liquid crystals. Dispersions of tiny droplets of liquid crystals in polymer indicate a characteristic electro-optical phenomenon provided the droplets of liquid crystals are smaller than the wavelength of the visible light. In such a case the arrangement of the molecules in a droplet changes under the influence of the external electric field, also causing a change in the appearance of the thin layer of such a mixture that becomes transparent above a certain value of the electric-field strength. A review of such literature can be found in the book by Paul S. Drzaic, Liquid Crystal Dispersions (World Scientific Publishing Company, Singapore, 1995). Reference literature includes no reports on the use of tiny droplets of cholesteric liquid crystals as an optical 3D Bragg-type microresonator that could be used as a 3D source of the laser light.